1. Field of the Invention
The invention relates to improved catalysts, methods of making the same and to processes of using the same for low temperature oxydehydrogenation of ethane to acetic acid and ethylene providing increased ethane conversion and higher selectivity to acetic acid.
2. Description of the Related Art
Documents are cited in this disclosure with a full citation for each appearing thereat. These documents pertain to the field of this invention; and, each document cited herein is hereby incorporated by reference.
The use of molybdenum and vanadium containing catalyst systems for low temperature oxydehydrogenation of ethane to ethylene has become known since the publication of "The Oxidative Dehydrogenation of Ethane over Catalyst Containing Mixed Oxide of Molybdenum and Vanadium" by E. M. Thorsteinson, T. P. Wilson, F. G. Young and P. H. Kasai, Journal of Catalysis, vol. 52, pp. 116-132 (1978). This paper discloses mixed oxide catalysts containing molybdenum and vanadium together with another transition metal oxide, such as Ti, Cr, Mn, Fe, Co, Ni, Nb, Ta, or Ce. The disclosed catalysts are active at temperatures as low as 200.degree. C. for the oxydehydrogenation of ethane to ethylene. Some acetic acid is produced as a by-product by these methods.
Several U.S. Pat. Nos. (4,250,346, 4,524,236, 4,568,790, 4,596,787 and 4,899,003) disclose low temperature oxydehydrogenation of ethane to ethylene. U.S. Pat. No. 4,250,346 discloses the use of catalysts of the formula Mo.sub.h V.sub.i Nb.sub.j A.sub.k in which A is Ce, K, P, Ni, and/or U, h is 16, i is 1 to 8, j is 0.2 to 10, and k is 0.1 to 5. The patent discloses one phosphorus-containing catalyst which is set forth in the examples having a Mo.sub.16 V.sub.4 Nb.sub.4 P.sub.4 oxide composition supported by silica-alumina. This catalyst is inactive in the oxidation of ethane at 320.degree. C. Furthermore, this patent reference is concerned with obtaining a high selectivity to ethylene, rather than acetic acid.
U.S. Pat. No. 4,454,326 is directed to the use of a calcined catalyst of the formula Mo.sub.a V.sub.b Nb.sub.c Sb.sub.a X.sub.e.
The above cited patents reference other patents concerned with the production of ethylene from ethane by the oxydehydrogenation process wherein acetic acid is formed as a by-product.
European Patent Publication EP 02 94 845 discloses a process for the higher selective production of acetic acid by the oxidation of ethane with oxygen in contact with a mixture of catalysts consisting of (A) a catalyst for oxydehydrogenation of ethane to ethylene and (B) a catalyst for hydration/oxidation of ethylene. The ethane oxydehydrogenation catalyst is represented by the formula Mo.sub.x V.sub.y Z.sub.z, wherein Z can be nothing or Nb, Sb, Ta, W and many other metals.
European Patent Publication EP 04 80 594 is directed to the use of an oxide catalyst composition comprising tungsten, vanadium, rhenium and at least one of the alkaline metals for the production of ethylene and acetic acid by oxidation of ethane with a molecular oxygen containing gas. The replacement of tungsten in whole or part by molybdenum carried out in EP 04 07 091 results in an increase in selectivity to acetic acid at the expense of the selectivity to ethylene.
EP 05 18 548 relates to a process for making acetic acid by oxidation of ethane in contact with a solid catalyst having empirical formula VP.sub.a M.sub.b O.sub.x, where M is one or more optional elements selected from Co, Cu, Re, Nb, W and many other elements, excluding molybdenum, a is 0.5 to 3, b is 0 to 0.1. The patent publication discloses that the catalyst contains a crystalline vanadyl pyrophosphate phase.
European Patent Publication EP 06 27 401 describes the use of a V.sub.a Ti.sub.b O.sub.x catalyst for oxidation of ethane to acetic acid. The catalyst composition may comprise additional components from a large list of possible elements.
It would be desirable to produce an improved catalyst for use in the oxidation of ethane to acetic acid with higher yields and increased selectivity of acetic acid.